Numerous designs have been developed and used over the past years incorporating various methods of providing a modulated power supply for an audio amplifier in order to improve efficiency and reduce dissipation of the power devices. Many prior art designs have disclosed either dual or multiple rails that switch to a higher voltage when the output swing of the amplifier is near clipping. These designs have been termed Class H and Class G where a secondary or multiple rail voltages are selected as required based on amplifier output swing. More complex designs have been realized with continuously variable rails that track the input signal and adjust the power supply rails so as to maintain a constant voltage between the output devices output swing and the power supply rail voltage. Most of these amplifiers are very complex requiring Pulse Width Modulation of the power supply and increased manufacturing requirements due to the large associated circuitry required to provide the tracking supply rails. Countless Class D designs have also been offered commercially which convert an input audio signal into a series of output pulses. When the pulses are averaged over time and low pass filtered to remove higher order harmonic information the output will be a replica of the input signal. While Class D offers the highest level of efficiency it is also one of the most difficult to use in applications where low EMI/RFI performance is required. While all of the various topologies have seen varying degrees of success commercially, the designs that offer the best cost vs. performance gain the widest market acceptance. Many of the numerous designs have excellent performance but may also be the most difficult to manufacture. At the same time high output automotive audio power amplifiers based on switch mode power supply technology has been available for years as aftermarket products but have not been embraced by the original equipment manufacturers (OEM) due to a number of undesirable side effects including switching transients which cause large levels of RFI emissions. In order to deliver high power automotive audio systems an efficient, high power DC to DC converter is required, which will convert the 12 volt automotive battery voltage to a higher supply voltage with high current output capability. The automotive charging system typically produces between 13.5 and 14.4 volts when the engine is running. While this is a slight increase above the 12 volt battery voltage this is not enough for high power amplification of audio signals. Amplifiers capable of high output power either need more voltage swing than the typical 14.4 volts available with the engine running or need to provide an extremely high current output in order to drive very low impedance loads. Typically, speakers with lower impedances have lower efficiencies and therefore a gain in output power with audio amplifiers that can deliver higher output current may not result in a large net gain in sound pressure levels. The formula to calculate output power is given by:
  POWER  =                    V        rms            ·              I        rms              =                            V          rms          2                R            =                        V          peak          2                          2          ⁢          R                    
An ideal power amplifier that can swing all the way to the power supply rails with a 14.4 volt supply can deliver a peak amplitude of 7.2 volts when connected to a 4 ohm load and would deliver 6.48 watts. Most automotive audio power amplifiers are dual amplifiers connected in what is termed “Bridge Mode” with one amplifier channel swinging positive and one swinging negative with the load connected between the two amplifier outputs and can, as a result, deliver twice the voltage swing across the load. This means that an ideal amplifier that can swing to the rails in bridge mode can deliver 14.4 volts peak which would deliver a total of 25.9 watts into a 4 ohm load. In reality, most power amplifiers are far from ideal and can typically only swing to within about 1.5 volts of the positive and negative power supply rails. As a result, the real world power amplifiers actual power output with the alternator running and 14.4 volts at the battery is closer to 16 watts. Thus it becomes obvious that the output of a car audio amplifier is limited by the voltage of the car battery with the alternator running. In most actual car systems, the amplifiers are connected in bridge mode configuration as described above, and speaker impedances are no higher than 4Ω, but it becomes apparent that the maximum output power per channel is roughly 30 watts even when driving a 2 ohm load and only about 16 watts with a 4 ohm load. High-power car amplifiers have been available for many years in the automotive aftermarket and these amplifiers use a DC-to-DC converter to generate a higher power supply voltage. In order to increase the battery voltage to a level capable of producing a higher power level most aftermarket automotive power amplifiers use switch mode power supplies SMPS to convert or transfer power from the 12 volt automotive battery (14.4 volts with the engine running) to a higher output voltage. While switching power supplies have seen improvements in terms of output power and efficiency even the best designs today produce an unacceptable level of radio frequency interference RFI and as a result have not seen wide acceptance for use in OEM vehicles. Other improvements in SMPS have been made offering higher switching frequencies, which allow component sizes to be reduced but produce even higher levels of RFI emissions. The common (SMPS) used in automotive aftermarket audio applications switches the battery voltage at a frequency between 25 kHz and 100 kHz to generate an AC square wave signal at the primary side of a step-up transformer. The stepped up waveform on the secondary of the transformer is rectified and filtered back to a DC signal. The output is typically a symmetrical +/−25 to 35 volts.
DC-DC converters based on charge pump or flying capacitor technology have been widely used in low power DC-DC converters but have seen limited use in high power applications due to a number of limitations including high pulse currents that occur at the switching transients which reduce efficiency and increase RFI problems. The low power circuits typically switch at higher frequencies between 20 KHz and 150 KHz which reduce the size of the capacitors but also contribute to an increase the RFI emissions. Integrated circuits have been produced for years based on the concepts of charge pump circuitry and have provided circuits which offer low current designs capable of delivering only milliamps of current to an external load. U.S. Pat. No. 5,066,871 is an example of one such design but many of the integrated circuit manufacturers offer IC's based on charge pump technology. There are many current offerings for low power switched capacitor technology in integrated circuit form from manufacturers including Analog Devices, Linear Technologies and National Semiconductor, to name just a few. However, none of these circuits can be used in a higher power application that can deliver amperes of output power required for automotive audio power amplification.
One recent prior art system offers improved switched capacitor technology by charging a capacitor to the supply voltage and switching the charged capacitor when additional output swing is required. In order to keep the amplifier output swing centered, a reference voltage is added to the input to switch the amplifier center bias when the additional capacitor voltage is switched on. One drawback to this system is the adding of undesirable switching transients in the output signal. While this system will double the power supply voltage when needed, it is also limited to two times the power supply voltage, i.e. 28 volts with a 14 volt supply, and therefore requires relatively low impedance drivers in order to gain large amounts of output power. This system operates as a class H or class G amplifier when the additional rail voltage is switched on and off, which improves dissipated output device heat but does not gain the full advantage of a tracking rail or adaptive rail design. A full tracking rail or adaptive rail design will provide even better reduction of dissipation. The implementation of a pulsed rail configuration will greatly reduce the heat dissipation of the power MOSFETs used in switching the supply voltage.
In another recent prior art system, a power amplifier is centered between the supply voltage and both the positive and negative rails are increased as the output of the amplifier requires more voltage swing. This system basically uses additional power amplifiers with a gain of 1 to track the audio power amplifier output at unity gain. The system monitors the audio amplifiers input or output swing and when a threshold is exceeded the power boost amplifiers will boost the power supply rails by driving charged capacitors between the output of the boost amplifiers and the power supply rails. This system does provide a tracking rail design which tracks the output swing of the audio power amplifier. However, the net gain in output power is relatively small, on the order of a few watts, compared to bridge mode designs. This is so because it is a single ended design. The second major drawback to this design is the complexity of the system, including the power boost amplifiers. In order to provide boost amplifiers with unity gain, a full amplifier circuit is implemented with a power MOSFET output stage. A relatively small gain in total system output power is achieved at the expense of increased cost and complexity in implementing this system. While the power boost amplifiers certainly can be fully integrated in IC form, reducing build complexity, the design requires large output MOSFETs and will therefore require expensive integrated circuit packaging that will provide some form of heat sink interface to keep the output devices cool. The prior art system also teaches using a gain slightly above unity in order to avoid capacitor voltage droop or sag due to the capacitor discharging under amplifier load. The amount of voltage droop or sag is difficult to measure in operation and, therefore, adding additional gain may be inadequate in some circumstances or may be excessive in others.
The low voltage method of controlling tracking rails in an audio power amplifier disclosed herein is well-suited to automotive applications, such as using a subwoofer channel at bass frequencies, where a maximum of 14.4V DC is available with the alternator running. However, because the loop gain of the control system is extremely high, it can in some applications cause overshoot of the control resulting in instability, particularly at frequencies above 3 KHz. Having a larger voltage differential between the amplifier output and the adaptive rail mitigates the instability, but may also reduce efficiency.
There is also a need for improvement of efficiency in professional audio applications that do not have the voltage limitations of the automotive applications. The maximum available power at a typical 120V/20 A AC wall socket is the product of 120V RMS AC and 20 A or 2400 VA. A professional audio amplifier connected to a typical AC mains circuit converts this available AC power into audio power to drive a loudspeaker. If the audio amplifier were 100% efficient, it could not deliver more than 2400 VA on a continuous basis. But even class D amplifiers do not reach 100 percent efficiency, most designs being closer to 90% and the best designs not achieving much better than 90% efficiency, decreasing the final power conversion factor of the system. Consider known professional audio applications in which audio amplifiers have positive and negative power supplies and theoretical 100% efficiency of both the power supplies and the amplifier. Assume the amplifier can deliver 2500 watts peak power or 100V peak signal swing into a 4 ohm load at 25 A to power the rails of the amplifier for both the positive and negative power supplies. The required peak output power of each power supply is, therefore, 2500 watts. Such a demand on the power supply is a severe limitation on the maximum power available from a typical 20 A RMS AC mains circuit.
Furthermore, “Watts RMS” is widely used by the audio community and has become an accepted, though inaccurate, way to rate audio amplifier output. While “RMS” is an appropriate term when applied to voltage or current, it is not appropriate to power. Rating audio amplifier output power in “Watts RMS” could only be valid if the output power were measured using a pure sine wave (RMS volts) signal driving a resistive load. Adding the inefficiencies of both the typical power supply and the audio amplifier circuit, the actual available audio power is considerably less than a consumer might expect based on a “Watts RMS” rating.
Many audio manufacturers promote their audio power in terms of peak power or “music power” or a number of other power rating methods. Peak power actually is twice the available RMS power and has become the most common rating for professional audio power amplifiers because of the apparently higher power rating. But the continuous RMS power available simply cannot exceed what is available from the AC mains circuit. Some manufactures avoid the issue completely by simply specifying “watts” and not specifying how the output wattage was measured.
It is, therefore, an object of the invention to provide an adaptive rail power amplifier using charge pump circuitry which overcomes the limitations of the above-mentioned prior art designs allowing use in OEM automotive vehicles. It is another object to provide an adaptive rail power amplifier with the ability to produce an output voltage swing above the input power supply voltage dynamically and as needed. In particular, it is an object of the invention to provide a power amplifier with adaptive tracking power supply rails offering greatly reduced complexity. It is a further object of the invention to provide an adaptive rail power amplifier with dynamically controlled charge pump converters for use in automotive power amplifiers capable of delivering well in excess of 100 watts of output power. It is a further object of the invention to provide an increase of up to 3 times the input supply voltage as required, to increase the amplifier output swing. It is a yet a further object of the invention to provide an adaptive rail power amplifier technology capable of high power output level for use in OEM automotive applications without causing problems in radio reception. It is still another objective of the invention to provide an adaptive rail power amplifier technology with higher efficiency and reduced heat dissipation. It is yet another objective of the invention to offer improved tracking of the power supply rails that can automatically adapt to any droop or sag voltage in the stored charge of the capacitor used to elevate the power supply rails.
It is also an object of the invention to provide an alternate embodiment of the adaptive rail power amplifier which has the same level of performance as the above described invention with reduced parts and complexity. It is a further object of the invention to provide an alternate embodiment with a simplified design which lends itself to full integration. It is another object of the alternate embodiment of the invention to require a single reference voltage for proper operation and signal tracking.
Furthermore, it is an object of the invention to provide an adaptive tracking rail audio amplifier which is stable at frequencies above 3 KHz. Another object of the invention is to provide an adaptive tracking rail audio amplifier with full bandwidth operation. Yet another object of the invention is to provide an adaptive tracking rail audio amplifier which reduces power consumption. It is also an object of the invention to provide an adaptive tracking rail audio amplifier which operates over the entire audio spectrum up to 20 KHz. A further object of the invention is to provide an adaptive tracking rail audio amplifier which operates without distortion or collapse of the control rails. It is a further object of the invention to provide an adaptive tracking rail audio amplifier which operates without increasing the difference voltage between the output swing of the amplifier and the power supply rails. And it is an object of the invention to provide an adaptive tracking rail audio amplifier with improved rail control circuit efficiency.